Degradation of SiC High-Voltage pin Diodes
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Degradation of SiC High-Voltage pin Diodes
Seoyong Ha and J.P. Bergman Abstract The recent discovery of forward-voltage degradation in SiC pin diodes has created an obstacle to the successful commercialization of SiC bipolar power devices. Accordingly, it has attracted intense interest around the world. This article summarizes the progress in both the fundamental understanding of the problem and its elimination. The degradation is due to the formation of Shockley-type stacking faults in the drift layer, which occurs through glide of bounding partial dislocations. The faults gradually cover the diode area, impeding current flow. Since the minimization of stress in the device structure could not prevent this phenomenon, its driving force appears to be intrinsic to the material. Stable devices can be fabricated by eliminating the nucleation sites, namely, dissociated basal-plane dislocations in the drift layer.Their density can be reduced by the conversion of basal-plane dislocations propagating from the substrate into threading dislocations during homoepitaxy. Keywords: compound semiconductors, degradation, defects, recombination-enhanced defect reaction, silicon carbide.
Introduction The field of SiC-based electronics is attracting growing attention as our society becomes more dependent on electrical power. We are expecting to see more efficient and reliable electric power control systems for power transmission and distribution and for all kinds of machinery, including various types of electric vehicles. SiC will also play an important role in future computing and communication devices that will require high power density and high thermal resistance. Some relatively mature SiC-based devices such as Schottky diodes and metal semiconductor field-effect transistors have been commercialized successfully in the last decade. However, at the same time, we have faced an unexpected problem in highvoltage bipolar devices, which are preferred for applications with blocking voltages over 3 kV.1,2 During the long-term operation of test diodes with a constant forward-current load, the voltage drop has been observed to increase abnormally, resulting in unstable forward-current–voltage characteristics.3 Recently, we seem to have found the solution to this problem.
MRS BULLETIN • VOLUME 30 • APRIL 2005
SiC pin Diodes and Their Degradation The structure of SiC bipolar high-voltage diodes can be described briefly as follows. A thick (30–100 m) low-doped drift layer (typically, n-type) is grown on a high-doped n-type conducting substrate by chemical vapor deposition (CVD). The p-type anode is formed by ion implantation or grown by CVD on the drift layer. Standard metal ohmic contacts for p-type and n-type SiC are formed on the anode surface and back side of the substrate. The diode area is defined by reactive ion etching of the anode. Lendenmann et al.3 reported that 4H-SiC pin diodes exhibited an irreversible increase in the forward-voltage drop during long-term operation. This is equivalent to the increase in the ON-state resist
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